1. Field of the Invention
This invention relates to a laser apparatus. This invention particularly relates to an external resonator type of laser apparatus, which has a high output power and which is used in order to record or read out information.
2. Description of the Prior Art
Lasers have heretofore been used as scanners, and the like, in various fields in order to read out information from media, on which the information has been recorded, or to record information on recording media. The lasers are required to have a higher output power. However, the output power of a single laser device is limited to a certain level. Therefore, laser apparatuses have heretofore been used, in which a plurality of laser devices are arrayed in a one-dimensional pattern or a two-dimensional pattern, laser beams radiated out of the laser devices are combined with one another, and a combined laser beam having a high output power is thereby obtained.
As such laser apparatuses, external resonator type of laser apparatuses have been disclosed in, for example, Japanese Unexamined Patent Publication No. 58(1983)-16350 and U.S. Pat. Nos. 4,813,762 and 4,649,351. For example, the external resonator type of laser apparatus, which is disclosed in Japanese Unexamined Patent Publication No. 58(1983)-16350, comprises a plurality of laser devices, which are located in a resonator structure constituted of a spherical lens and a plane mirror, and a spatial mode filter, which is located in the resonator structure and which attenuates the modes other than the lowest-order transverse mode. With the interference effects given by the spatial mode filter, the laser beams having been produced by the plurality of the laser devices are combined with one another and radiated with the same phase, and a combined laser beam having a high output power is thereby obtained.
With the technique disclosed in U.S. Pat. No. 4,649,351, it is necessary to use a special phase grating for splitting each of a plurality of laser beams, which are to be combined with one another, into the same number of laser beams as that of the plurality of the laser beams, which are to be combined with one another, such that the split laser beams may have an equal intensity. It is necessary for the phase grating to be designed optimally such that the intensity distribution of the laser beam having been combined on the phase grating may become uniform and the beam combining efficiency may thereby be kept high. Therefore, if the number of the laser beams to be combined with one another becomes large, it will become difficult to design the phase grating optimally such that the intensities of the split laser beams may be kept equal to one another, and such that the intensity distribution of the laser beam having been combined on the phase grating may become uniform. Thus the disclosed technique has the drawbacks in that the beam combining efficiency cannot be kept high.
With the technique disclosed in U.S. Pat. No. 4,813,762, if the number of the laser beams to be combined with one another becomes large, the discrimination between the oscillation eigen modes (super modes), in which the laser beam is radiated, will become low, and therefore the laser beam will become apt to be radiated in a plurality of kinds of eigen modes. Accordingly, it will become difficult to radiate the laser beam only in the eigen modes, in which the laser beam is radiated with the same phase. Such problems have also been pointed out in, for example, "Modal Analysis of Linear Talbot-Cavity Semiconductor Lasers" by David Mehuys, William Streifer, Robert G. Waarts, and David F. Welch, OPTICS LETTERS/VOL. 16, NO. 11/JUNE 1991.